Re: [PATCH] xen/arm32: Get rid of __memzero()

[Michal Orzel <michal.orzel@xxxxxxx>]

On 27/11/2024 11:55, Julien Grall wrote:
> 
> 
> From: Julien Grall <jgrall@xxxxxxxxxx>
> 
> All the code in arch/arm32/lib/ where copied from Linux 3.16
> and never re-synced since then.
> 
> A few years ago, Linux got rid of __memzero() because the implementation
> is very similar to memset(p,0,n) and the current use of __memzero()
> interferes with optimization. See full commit message from Linux below.
> 
> So it makes sense to get rid of __memzero in Xen as well.
> 
>     From ff5fdafc9e9702846480e0cea55ba861f72140a2 Mon Sep 17 00:00:00 2001
>     From: Nicolas Pitre <nicolas.pitre@xxxxxxxxxx>
>     Date: Fri, 19 Jan 2018 18:17:46 +0100
>     Subject: [PATCH] ARM: 8745/1: get rid of __memzero()
> 
>     The __memzero assembly code is almost identical to memset's except for
>     two orr instructions. The runtime performance of __memset(p, n) and
>     memset(p, 0, n) is accordingly almost identical.
> 
>     However, the memset() macro used to guard against a zero length and to
>     call __memzero at compile time when the fill value is a constant zero
>     interferes with compiler optimizations.
> 
>     Arnd found tha the test against a zero length brings up some new
>     warnings with gcc v8:
> 
>       https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82103
> 
>     And successively rremoving the test against a zero length and the call
>     to __memzero optimization produces the following kernel sizes for
>     defconfig with gcc 6:
> 
>         text     data     bss       dec       hex  filename
>     12248142  6278960  413588  18940690   1210312  vmlinux.orig
>     12244474  6278960  413588  18937022   120f4be  vmlinux.no_zero_test
>     12239160  6278960  413588  18931708   120dffc  vmlinux.no_memzero
> 
>     So it is probably not worth keeping __memzero around given that the
>     compiler can do a better job at inlining trivial memset(p,0,n) on its
>     own. And the memset code already handles a zero length just fine.
> 
>     Suggested-by: Arnd Bergmann <arnd@xxxxxxxx>
>     Signed-off-by: Nicolas Pitre <nico@xxxxxxxxxx>
>     Acked-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
>     Acked-by: Arnd Bergmann <arnd@xxxxxxxx>
>     Signed-off-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxx>
> 
> Origin: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git 
> ff5fdafc9e97
> Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
In case you need Arm's ack apart from Jan's Rb:
Acked-by: Michal Orzel <michal.orzel@xxxxxxx>

~Michal